1. Field of the Invention
The present invention generally relates to a dynamic focus circuit of a display apparatus, and more particularly to a dynamic focus circuit of electromagnetic focusing type, which is used for a CRT (Cathode Ray Tube) of a projection television etc.
2. Description of the Related Art
The multi-media and the projection television are being popularized, by which a video image can be enjoyed at home on a wide picture plane with force. In order to display a video image source of high quality such as a high definition television, a clear vision etc., and display the character information and the drawing information in the multi-media, there is a tendency that the picture plane is getting larger and the resolution is getting higher. On such a background, it is demanded to improve the focusing capability in view of improving the image quality on the overall picture plane.
In the projection television, the video image of the CRT is magnified and projected by use of a projection lens. The focusing technique by means of a main focusing lens, is such a technique to focus an electron beam emitted from an electron gun to a smallest spot on a fluorescent surface of the CRT and on the screen, at this time.
There are two types of focusing method, i.e. an electrostatic focusing method, which adjusts focusing by adjusting the application of the voltage by combining the cylindrical electrodes within the CRT, and an electromagnetic focusing type focusing method (hereinbelow, it is referred to as an electromagnetic focusing method) which uses a focusing coil and the permanent magneto at the external of the CRT. The electromagnetic focusing method can perform such a control to make the spot diameter smaller than the electrostatic method, so that the electromagnetic focusing method is suitable for the high image quality projection television.
FIG. 7 shows a principle of the electromagnetic focusing method.
FIG. 7 shows a CRT 12 and a screen 11. The CRT 12 projects a video image onto the screen 11. A focus coil L1 for generating a correction magnetic field, and a permanent magneto 13 for generating a stationary magnetic field are equipped at a neck portion 12b of the CRT 12. An electron gun 14 is provided in the internal space of the CRT 12. An electron beam 15 is emitted from the electron gun 14.
At the central portion of a fluorescent surface 12a, the electron beam 15 is focused with a focal distance L from the electron gun 14 to the fluorescent surface 12a. At the peripheral portion of the fluorescent surface 12a, the electron beam 15 is deflected and is focused with the distance L'. In the figure, a curved plane is indicated by a broken line, which distance from the electron gun 14 is equal to the focal distance L of the central portion of the fluorescent surface 12a. At the peripheral portion, since the traveling distance of the electron beam 15 is the distance L', which is longer than the focal distance L, such a condition occurs where the focusing condition is slightly lost and the spot shape becomes oval shape. Therefore, there is required a technique to obtain the optimum spot condition. This can be achieved by changing the current flowing through the focus coil L1 of the CRT 12 in a parabola manner in correspondence with the deflecting current. This technique is a so called dynamic focusing method.
Some examples of the electromagnetic focusing type dynamic focus circuit are explained here with referring to FIG. 8 and FIG. 9.
FIG. 8a shows a driving circuit of the dynamic focus circuit of electromagnetic focusing type by means of a constant current amplifier. FIG. 8b shows the current I and the voltage V of the constant current amplifier, wherein they are called parabola wave because of their shape. The voltage VW for focusing, which is generated by a waveform generator 16, is amplified in current through a preamplifier 8 by an output constant current amplifier 17 for current driving, and is applied to a focus coil L. The voltage generated by a detection resistor RL for current-voltage waveform feed back loop, is compared with the input voltage VW by the preamplifier 8, to control the output constant current amplifier 17. By this, a current IL which is flowing through the focus coil L, becomes the parabola waveform as shown in FIG. 8b.
FIG. 9a shows a construction of the dynamic focus circuit of electromagnetic focusing type by means of a PWM (Pulse Width Modulation) method. FIG. 9b shows a signal waveform of each portion of the dynamic focus circuit of FIG. 9a.
The current waveform obtained by a detection resistor RL for current-voltage waveform feed back loop, is compared with the horizontal parabola wave of the voltage VW as shown in FIG. 9b, which is generated by the standard waveform generator 16, by an error amplifier 8a. The error amplifier 8 generates an error signal. The carrier component included in this error signal is removed at a low pass filter (LPF) 20, and is inputted to a comparator 19 as the error signal VE. If the voltage VW is a parabola wave, the error signal VE becomes a sawtooth wave. By constructing a general PWM generating means by a standard oscillator 18 and the comparator 19, a PWM signal VP is generated from the error signal VE. Then, the amplifier 17 is driven by the PWM signal VP, the parabola wave current L.sub.l1 can be supplied to the focus coil L1.
Other than the above explained examples, there are several dynamic focusing circuits of the electromagnetic focusing type. For example, each of Japanese Patent Laid Open Hei.3-53,671 (1991), Japanese Patent Laid Open Hei.4-271,695 (1992), Japanese Patent Laid Open Hei.4-70,181 (1992) and Japanese Patent Laid Open Hei.5-37,799 (1993), discloses a dynamic focus circuit.
However, the above explained dynamic focus circuit by means of the constant current amplifier has a problem that the electric power consumption is high. Namely, most of the electric power supplied from the electric source is consumed by the constant current output circuit. The electric power is as much as 70 to 80 Watt! in general, so that the heat generated by this consumed electric power causes an economical problem, a problem to give the room for the heat generation in the apparatus, and a problem that the increase of the power consumption of the overall television apparatus is greatly affected. For example, since the overall power consumption of the projection television is in a range from 180 to 260 Watt! in general, as the power consumption of only one control circuit with respect to this figure, the load of the power consumption is too heavy.
Further, in the above explained dynamic focus circuit of the PWM type, the problem of the power consumption can be improved to some degree, by employing the switching method for the current application by the constant current amplifier. However, in this case, since the current flows through the focus coil by the switching operation of several tens times of the horizontal synchronization frequency, a drawback such as an unnecessary irradiation etc., is caused by the switching noise.
On the other hand, there is a multi-sync display to display images which correspond to the standards of various computers. This multi-sync display is a display which can vary the horizontal scanning frequency f.sub.H in a range from about 15 KHZ to more than 75 KHz. Here, the power consumption and the unnecessary irradiation are greatly related to the horizontal scanning frequency f.sub.H. Thus, in order to adapt the above mentioned dynamic focus circuit of electromagnetic focusing type to the multi-scanning operation of the multi-sync display, the reduction of the power consumption and the reduction of the unnecessary noise generation are greatly desired.